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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Experimentelle Elementarteilchenphysik

Humboldt-Universität zu Berlin | Mathematisch-Naturwissen­schaft­liche Fakultät | Institut für Physik | Experimentelle Elementarteilchenphysik | ATLAS | Theses | Development and Deployment of an Inner Detector Minimum Bias Trigger and Analysis of Minimum Bias Data of the ATLAS Experiment at the Large Hadron Collider

Regina E Kwee (2012)

Development and Deployment of an Inner Detector Minimum Bias Trigger and Analysis of Minimum Bias Data of the ATLAS Experiment at the Large Hadron Collider

PhD thesis, Humboldt-Universität zu Berlin.

Soft inelastic QCD processes are the dominant proton-proton interaction type at the LHC due to their large production cross-section. More than 20 of such collisions pile up within a single bunch-crossing at ATLAS, when the LHC is operated at design luminosity of L = 1034 cm−2s−1 colliding proton bunches with an energy of √s = 14 TeV. These inelastic interactions are characterised by a small transverse momemtum transfer. Theoretically, they can only be approximated by phenomenological models which need experimental data as input for a more accurate description. During the initial phase of LHC beam operation in 2009, the luminosity ranged from L = 1027 to 1031 cm−2s−1 and the probability of a proton-proton interaction per bunch-crossing was as low as 0.1 %. This was an ideal period to select single proton-proton interactions and study general aspects of these soft inelastic processes.

In the first part of this thesis, a Minimum Bias trigger was developed and used for data-taking in ATLAS. This trigger, mbSpTrk, processes signals of the silicon tracking detectors of ATLAS and was designed to fulfill several requirements. The main functionality is that empty events are efficiently rejected, while possible biases in the selection of proton-proton collisions is reduced to a minimum. Several options and fall-back solutions were investigated and implemented to get a handle of the hits formed by electronic noise in empty bunch-crossing events, exploiting detector specific quantities like pixel cluster time-over-threshold and hit coincidence of silicon strips. The deployment of the trigger considered also changing background conditions for which a new configuration of online low-pT tracking was derived that allowed to retain low-pT events while machine background is highly suppressed.

ATLAS has as well a minimum bias trigger based on Minimum Bias Trigger Scintillators at disposal. The complementary nature of mbSpTrk allowed to study the performance of both triggers, their efficiencies and biases from the first collision data taken in 2009 and 2010. Both triggers showed a selection efficiency close to 100 % in the analysed phase-space regions.

These studies were then used in the second part of the thesis, where measurements of inelastic charged particles were performed in two phase-space regions. Charged particles were considered which were centrally produced with a pseudo-rapidity |η| < 0.8 and a transverse momentum of pT > 0.5 or 1 GeV. Four characteristic distributions were measured at two centre-of-mass energies of √s = 0.9 and 7 TeV: the charged particle multiplicity, their kinematic spectra in pseudorapidity and transverse momentum and correlations between the average pT and the number of charged particles. These results corrected back to hadron level are presented with minimal model dependency to compare them to predictions of different Monte Carlo models for soft particle production.

This analysis is also the ATLAS contribution for the first common LHC analysis to which the ATLAS, CMS and ALICE collaborations agreed. The pseudorapidity distributions for both energies and phase-space regions are compared to the respective results of ALICE and CMS.